Electric micrometer



pr 1 1, 1939 H. c. TURNER ET AL 2,154,156

I 1 ELECTRIC MICROMETER Filed July 6, 1957 Fig.1. B A c 1 Fig. 2.

' arc/Wafer Patented 11, 1939 UNITED sTATEs PATENT. OFFICE ELECTRIC MICROMETER Application July 6, 1937, Serial No. 152,202 In Great Britain July 6, 1936 5 Claims.

This invention relates to electric micrometers for measuring or indicating the thickness of metal foil and the like of the type in which the foil is introduced between a transmitter, gener- 5 ating electric oscillations, and a receiver, thereby producing modifications of the signal received by the receiver which are an indication of the thickness of the foil. i The object of'this invention is to improve electric micrometers of this known type, more particularly in the following respectsz-(i) reduction of errors due to variation in the-position of the foil, (ii) increase of sensitivity or ability to detect small changes in thickness, (iii) protec- 5 tion of the apparatus from overloads, (iv) reliability, (v) simplicity. In the accompanying drawing, Figure 1 is a diagram of the usual electric micrometer circuit; Figure 2 is adiagram of a compensated electric 20 micrometer circuit including opposed bridge rectiflers and Figure 3 is a more complete diagram of the circuits of the improved electric micrometer.

We have found that an essential condition for 25 attaining (i) is that the transmitter and receiver should be symmetrical. Thus in Figure 1 of the accompanying drawing, which shows, diagrammatically the arrangement usually employed in" electric micrometers, E being the oscillation gen- 30 orator, it is important that the components L1,

L2, C1, (Ia-R1, R2 should be related by the equation 35 R1 and R2 are to be taken to represent the total effective shunt resistance in each circuit. Symvmletry in respect of the L's and Us has probably been obtained before; but the need for symmetry in respect of the R's appears not to have been recognised. Although ideally the foil should be positioned mid-way between the transmitter and receiver coils, that is in the position indicated aty ii, no appreciable errors will be introduced if this position is shifted as for example 45 to position B or C.

It should be explained that the two circuits, comprising L1, C1, R1, and L'zfC'z, Rafare so arranged -that upon introducing the foil' whose thickness is to be measured between the coils of the circuits the mutual inductance between these coils is changed and the effective resistance of eachcircuit is increased. Provided that the range of movement of the foil is small, the mutual inductance remains substantially independent of the 1011's position and dependent, -i'or a given material, on the thickness of the foil. In practice the movement of the foil away from a central position is limited to, say plus or minus V th inch, and the gap between the coils is to 4; inch. The e'ilfect of movement of the foil is to cause 5 changes in the effective resistance value of the coils. The values of R1 and R: can be adjusted so that there is practically no change of output when the foil is moved within the gap between the coils. It is then found that the values of the 10 various components of the circuits satisfy the relationship given. Actually in practice the val-.

ues of the effective resistances are adjusted until there is substantially no deflection of the needle of the indicating instrument when the foil is moved within the above mentioned limits between the coils.

According'to-the invention an electric micrometer is electrically symmetrical in the sense just explained.

Object (ii) is known to be promoted by using compensation methods, in which the disturbance in the receiver associated with one transmitter by the presence of the foil is compensated by the disturbance produced in another similar receiver associated with another similar transmitter by the presence of some calibrated'object, such as a movable vane. We have found that, in such a compensated arrangement, the outputs of the two receivers are best applied to a pair of opposed bridge rectiflers, the out-of-balance D. C. current being applied to a galvanometer. Such 'an arrangement is shown in Figure-2 of the accompanying drawing. Here a common input E feeds two circuits, one associated with the foil to be examined and the other with a calibrated vane, the latter forming the compensating circuit. The outputs of the two receivers are ap- "plied to 'the pair of opposed bridge rectiiiers in the manner shown. Since the D. C. terminals of 40 the rectifiers are connected in opposition, each rectifier tends to shunt the D. C. output of the other rectifier. This shunting effect is negligible under normal conditions, thatis when the outof-balance current is small, so that it does not affect the sensitivity. When the out-of-balance current is larger, the resistance of the shunting rectifier decreases and shunts a larger part of the current, thus protecting the galvanometer G; a resistance R: is provided and its value is adiusted suitably to control the current atwhich this shunting eifect comes into operation; accordingly the sensitivity of the apparatus can be regulated by its use.

In this figure, R1 and R2 are to be taken also v a current-measuring instrument.

The remaining objects (iii), (iv), (v) are promoted by using dry-plate rectiflers as the rectifle'rs.- For these are cheap, simple and stable. According to a subsidiary feature of theinvention, the rectiflers in a micrometer possessing the second of the said features are dry-plate rectiflers.

Figure 3 of the accompanying drawing shows the general arrangement of an electric micrometer embcdying'all'the desirable features aforesaid. It requires no detailed explanation, except perhaps to make clear that the resistances and/or capacitances may be adjustable in order to allow for correction of the relation given above.

We claim:-

1. An electric micrometer comprising means .including a primary inductor coil having a resistor in series therewith and a condenser across the terminals thereof, the total resistance of each of the transmitter circuits being substantially the same, two receiver circuits arranged for cooperation with said transmitter circuits respectively, each receiver circuit comprising a sec ondary inductor coil having a resistor in 'series therewith and a condenser across the terminals thereof, the total resistance of each of the receiver circuits being substantially the same and in each receiver circuit the total resistance being in predetermined ratio to the total resistance of the corresponding transmitter circuit, the 00-- operating primary and secondary coils of the respective transmitter and receiver circuits being spaced apart one to receive therebetween'the foil to be tested and the other a calibrated vane, a bridge rectifier in each of said receiver circuits, circuit connections between said bridge rectifiers arranged so that the direct currents of the rectiflers are in opposition and a galvanometer connected across said circuit connections formeasuring the direct current difler- .ence between said rectiflers.

2. An electric micrometer as in claim 1 wherein the total resistances of the cooperating trans-' mitter and receiver circuits are directly proportional to the inductance of, the primary and sec- I ondary coils and inversely proportional to the' capacities of the condensers across said primary and secondary coils.

3. An electric micrometer as in claim 1 wherein said bridge rectifiers comprise dry-plate rectiflers. 4. An electric micrometer as in claim 1 wherein a resistor is provided in circuit with said galvanometer to protect the latter from excess current.

5. An electric micrometer as in claim 1 wherein said bridge rectiflers comprise dry-plate rectiflers subject to reverse current discharge when said current difference eirceeds a predetermined amount, said reverse discharge being adapted to protect the galvanometer, and a resistor in circuit with the galvanometer is adapted to limit the current to the galvanometer.

, HENRY COBDEN TURNER.

THOMAS CAY'I'ON NUTIALL. 

